1. Field of the Invention
This invention relates to the field of switched regulated power supplies often referred to as voltage-step-up/current-step-up dc-to-dc converters.
More particularly, this invention relates to improved regulation of the output voltage of such switched regulated power supplies.
Even more specifically, this invention relates to the elimination of the adverse effects of leakage flux associated with the energy-storage transformer in a dc-to-dc converter.
2. Prior Art
A familiar use of switched power supplies was in the early vacuum tube which operated automobile radios. The audible hum noted when the radio was first switched on was that of a vibrating switch which "chopped" the dc voltage applied to the primary of a transformer. This on-and-off again, dc excitation of the primary gave rise to an ac voltage in the secondary of the transformer. This secondary ac was rectified and applied as dc operating voltages to the various vacuum tube circuits of the radio.
The principle of chopping the dc primary input to a transformer and rectifying the ac componant of the secondary remains valid today. Transistor switches and solid state diodes have generally replaced the mechanical choppers and the rectifiers of the early auto radio. Modern day converters may generally be found in one of three generally used configurations well known to those skilled in the art. These are: the flyback converter, the forward converter, and the push-pull converter.
Frequently dc-to-dc converters have multiple outputs. One of these is denoted the main output and the others as auxiliary outputs. A controller is generally provided to regulate the output voltage of the main output. Because of the mutual inductance existing among the windings of a transformer, the effect of regulation of the main output is to also regulate, to some extent, the auxiliary outputs as well. This regulatory effect on the auxiliary outputs is denoted as "cross-regulation". The effectiveness of cross regulation is degraded by inherent characteristics of the circuitry such as resistance drops in the transformer windings, forward voltage drops across diode rectifiers, and leakage inductance of the transformer.
Leakage inductance results from the fact that all lines of flux within the transformer do not cut across every winding of the transformer. Leakage inductance leads to a tendency, in cross-regulated auxiliary outputs, for the auxiliary output voltage to rise significantly above the value to which it would be ideally regulated under light load conditions. So too, there is also a noted tendency for the auxiliary output to sag below its ideally cross regulated output under heavy load conditions. To overcome these adverse effects, linear regulators using pass transistors are often used to regulate the auxiliary outputs. Such linear regulators do not provide an efficiency-effective result since such pass transistors are inherently dissipative.
It is therefore an object of the invention to provide an essentially non-dissipative means of regulating auxiliary outputs of dc-dc converters.
It is a further object of the invention to provide supplementary regulation of auxiliary outputs of dc-dc converters so as to preserve the isolation existing among outputs of said converter.
It is a even more specific objective of the invention to provide a means for regulation of auxiliary outputs of such converters which means permits the construction of the transformer to be simplified.